Magnetic beta-ray spectrometer and magnetic lenses for use therein

ABSTRACT

A magnetic beta-ray spectrometer, in which the axial field patterns of the collimating and focusing lenses undergo synchronous and identical variations, thereby providing for an image magnification equal to unity.

United States Patent Gumenjuk et a1.

[54] MAGNETIC BETA-RAY SPECTROMETER AND MAGNETIC LENSES FOR USE THEREIN inventors:

Filed:

Appl. No.:

U.S.Cl

Int.Cl.

Boris Andreevich Gumerriuk, Nikitinskaya ulitsa, 26, korpus l, kv. 37, Moscow; Boris Petrovich Peregud, Yakovlevsky pereulok, 3, kv. 4, Leningrad; Leonid Cherstvov, shosse Entuziastov, 190/1, kv. 301, Moscow; Georgy Ljubimovich Granberg, Ananievsky pereulok, 5, kv. 133, Moscow; Galina Davydovna lvanova, Zelenodolskaya ulitsa, l7, korpus 5, kv. 71, Moscow, all of USSR.

Sept. 19, 1968 ..250/41.9 ME, 250/495 D, 335/210 .1101 j 39/34 [151 3,659,095 [451 Apr. 25, 1972 [58] Field ofSearch ..250/41.9,49.5, 106; 335/210, 335/213 [56] References Cited UNITED STATES PATENTS 3,461,306 8/1969 Stout et al ..250/41.9

Primary Examiner-James W. Lawrence Assistant Examiner-A. L. Birch Attorney-Waters, Roditi, Schwartz & Nissen [57] ABSTRACT A magnetic beta-ray spectrometer, in which the axial field patterns of the collimating and focusing lenses undergo synchronous and identical variations, thereby providing for an image magnification equal to unity.

2 Claims, 6 Drawing Figures PAIfmEmmsmzz 3.659.095 SHEET 2 OF 3 MAGNETIC BETA-RAY SPECTROMETER AND MAGNETIC LENSES FOR USE THEREIN This invention relates to improvements related to magnetic beta-ray spectrometers intended for determining the energy distribution of electrons emitted by a radioactive source.

Known in the art are beta-ray spectrometers wherein use is made of two magnetic lenses and a magnetic prism or some other dispersing element, one of the lenses being a collimating lens which converts the diverging beam of beta-rays from a source into a parallel beam. 1

The beta rays of the resultant parallel beam emerge and pass through a dispersing element, e.g. a magnetic prism, undergo dispersion as to energy and thereupon are directed to the second magnetic lens which focuses the beta rays onto a receiver.

These prior art beta-ray spectrometers can be operated so as to provide for high resolving power or for maximum utilization of the solid angle available from the source. To adjust the beta-ray spectrometer from one operating mode to another, it is pertinent to vary the focal length of the collimating lens which operation necessitates displacing the radioactive source.

Where recourse is had to diminishing the focal length of the collimating lens in order to provide for maximum utilization of the solid angle of beta-rays emission, the electron-optical image in the focal plane of the focusing lens in some instances exceeds the dimensions of the receiver, so that there ensue singificant luminance losses.

In order to minimize spherical aberration under high resolution operating conditions, the axial pattern of the magnetic field of the lenses of the prior art beta-ray spectrometers should be as flat as possible, whereas enhanced utilization of the solid angle available from the source calls for a maximum steepness of the magnetic field axial pattern.

Selection of a specific field pattern is invariably a compromise between two extremes and, generally is incapable of providing for a maximum solid angle or for a minimum spherical aberration under high resolution operating mode.

It is to be noted that where use is made of a large solid angle, i.e. when the beta-ray spectrometer can best be used under beta-gamma coincidence operating mode, the source will be close to the center of the lens magnetic field pattern, thereby excluding the possibility of disposing appropriately a photocell (receiver) and detector.

The known beta-ray spectrometers present substantial difficulties in operation, insofar as varying the focal length of the collimating lens necessitates the displacement of the radioactive source, other pertinent manipulations being the adjustment of the radioactive source rotation angle as well as of the collimating lens current.

It is an object of the present invention to eliminate the aforesaid disadvantages inherent in the prior art beta-ray spectrometers.

A further and more specific object of the present invention is the provision of a beta-ray spectrometer in which the lenses make it possible to obtain, under all operating modes, an electron-optical magnification of the image which equals unity, and also make it possible to obviate the necessity of adjusting the spectrometer for change-over from one operating mode to another.

This object of the invention is accomplished by a beta-ray spectrometer comprising a magnetic collimating lens having an electric winding and intended for shaping a parallel betaray beam, a magnetic element which effects the dispersion of beta rays as to energy, and a magnetic lens with electric windings which focuses the beta rays of a specific energy onto a detector, wherein, according to the invention, the electric windings of both magnetic lenses are made so that their magnetic fields exhibit synchronous and identical variations of axial patterns, and at the points, where the focal length of each lens remains at a minimum for any magnetic field distribution pattern, are placed a beta-ray source and a detector, respectively.

It is good practice to make the electric windings of each lens in the form of electrically insulated sections disposed along the optical axis of the lens, each section comprising a plurality of conductive layers insulated electrically from each another and capable of being connected in different combinations, which arrangements are instrumental in providing for a variable axial pattern of the lens magnetic field.

The beta-ray spectrometer, according to the present invention, is advantageous in that it provides for the equality of the focal lengths of thecollirnating and focusing lenses under all operating modes, so that magnification of the source image in the focal plane of the focusing lens can be avoided, thereby rendering possible maximum utilization of the instrument luminance.

Insofar as the radioactive source and the detector are disposed at the centers of the lens field patterns, it is superfluos to adjust the angle of radioactive source and detector rotation for each operating mode variation since due to this arrangement of the source and detector the rotation of the image caused by the magnetic field of the lens will remain constant for all field patterns.

Other objects and advantages of the present invention will become apparent from a consideration of the description of exemplary embodiments thereof with reference to the accompanying drawings, wherein: 1

FIG. 1 is a schematic diagram of the beta-ray spectrometer;

FIG. 2 is a general view of an embodiment of the beta-ray spectrometer magnetic lens which can be employed either as a collimating or a focusing lens;

FIG. 3 is schematic diagram of the cross-sectional area of a quarter of the magnetic lens shown in FIG. 2;

FIG. 4 is a shortened magnetic collimating lens with a radioactive source disposed therein;

FIG. 5 shows graphically the dependence of the utilization of the solid angle available from the source upon the position of the source on the collimating lens axis; and

FIG. 6 is the axial pattern of the magnetic field of the shortened magnetic lens.

It is known to employ in prism-type beta-ray spectrometers magnetic lenses exhibiting a bell pattern of the magnetic field, wherein the field strength along the lens axis varies, according to the following equation M wherein H, is the magnetic field strength on the axis at the lens center;

1 denotes the distance from the lens center to the point of field strength measurement, and

a is the distance from the lens center to the point where the field strength is half as great as that in the lens center.

Experiments and relevant calculations show that increasing the magnitude of a essentially diminishes the aberration and, hence, improves the resolving power of the beta-ray spectrometer and also results in varying the solid angle available. It follows from curve A (FIG, 5) on which the solid angle used is plotted versus the position of the source on the lens axis that for any value of a the solid angle attains a maximum value when z i 0, Le. in the case where the focal length of the lens is at a minimum and equals a.

Hence, the optical axis of the lens incorporates a point at which the focal length is at a minimum for any value of a, said point being coincident with the lens center.

The beta-ray spectrometer accommodates collimating lens 1 (FIG. 1), at the center of which there is disposed a radioactive source 2, magnetic prism 3, which disperses a parallel bearn emerging from lens 1 according to the energy of the beam, and focusing magnetic lens 4, in the center of which lens 4 there is located receiver slit 5, the electron detector 6 being located behind said slit.

Collimating lens 1, which is identical in lens 4, comprises vacuum tube 7 (FIG. 2),

design to focusing in which provision is made for external winding 8 closed with magnetic shield 9. (Calculation of the distributively wound lens winding can be performed with the conventional formulae which are given, for instance, in the article Short magnetic lens with a distributed winding by Kelman V.M., Peregud B.P., Skopina V.l., Jh.T.F., 29, 1219 /l959/. Calculation of the magnetic fields of the lenses is given in the book Grundlagen der Elektronenoptik by Dr. Walter Glaser, Wien, Springer-Verlag publishing house, 1952). Winding 8 (FIG. 3) consists of sections 10, ll, 12 and 13 electrically insulated from each another and located along the optical axis of the lens. Each section contains a different number of conductor layers insulated from each another, each layer being furnished with leads is connected to a commutating device (not shown in the drawings) intended for energizing the layers and sections of the winding in specified combinations.

The aforesaid design of the lenses makes it possible to generate thereinside a magnetic field having a variable axial pattern or, to state it differently, to vary the parameter a, thereby providing the possibility of switching over from the high resolving power operating mode to the operating mode making use of large values of the solid angle. In view of the fact that the radiation source and receiver are disposed at the lens centers, any value of a under the high resolving power operating mode is provided for the utilization of a maximum solid angle.

In the present beta-ray spectrometer the sections of the windings of both lenses 1 and 4 are coupled through a switch (not shown in the drawings) to a common stabilized power supply source (not shown in FIGS). The sections of both lenses, identically made and arranged, are synchroniously energized by means of the aforementioned switch so that variations in the magnetic field pattern of said lenses occur simultaneously. With this arrangement the magnification of the electron-optical image is" invariably equal to unity and the instrument luminance, when recourse is had to large solid angles, is significantly larger than that attainable in the prior art beta-ray spectrometers.

In view of the fact that in the present beta-ray spectrometer the radiation source and the detector are disposed at the lens centers, it is expedient to shorten the windings of both lenses, as shown in FIG. 4, thereby facilitating access to the radiation source and making it possible to mount a scintillation crystal with photomultiplier proximate the source in question. The employment of shortened windings does not adversely effect on spectrometer performance, inasmuch as the beam does not traverse the magnetic field in that part of the lens where the winding is absent, the effect of missing turns of the winding on the magnetic field in the working portion of the lens being insignificant and incapable of enhancing the aberration,

although there does take place a slight deviation of the magnetic field pattern from a symmetrical bell-type shape (cf. Curves B and C in FIG. 6).

The present beta-ray spectrometer is superior to the prior art instruments in that it provides the possibility of employing much smaller detectors and counters, this being due to fact that the image size is invariably equal to the radiation source size. The equality of image and source sizes results, in turn, in diminishing the effect of the natural background and also obviates the necessity of varying the dimensions of the receiver slit when changing over from one operating mode to another.

The employment of the present beta-ray spectrometer provides at least a 10-fold gain in luminance at solid angles of greater than 3 percent of 47.

Although the present beta-ray spectrometer has been described with reference to the preferred embodiment thereof, it is apparent that various alterations, modifications and changes can be made therein without departing from the spirit and scope of the invention as disclosed in the appended claims.

We claim:

l. A magnetic beta-ray spectrometer comprising a collimating ma netic lens, a radiation source means for supporting the ra ration source ad acent the col im'ating magnetic lens, said collimating magnetic lens including, electric windings for directing beta-rays which emerge from said radiation source in a parallel beam; a focusing magnetic lens, a detector, means for supporting the detector adjacent the focusing magnetic lens, said focusing magnetic lens including electric windings for focusing the beta-rays'onto said detector, the windings of both said lenses being adapted for producing respective focusing magnetic fields, said magnetic fields of both said lenses having respective axial patterns being synchronous and of identical variation, said radiation source and. said detector being disposed separately in the respective lens thereof within zones in which the focal length of each respective lens is preserved at a minimum for each axial pattern of each magnetic field; and a magnetic element interconnecting each said lens for dispersing the beta-rays of the parallel beam according to energies of said parallel beam and directing the dispersed beta-rays onto the detector of the focusing lens.

2. A beta-ray spectrometer according to claim 1, wherein the respective windings each includes a plurality of separate sections electrically insulated from each other, said windings being disposed along the optical axis and further including a plurality of conductive layers electrically insulated from each other, said sections and said layers being coupled to a power supply source for producing a variable axial pattern of the magnetic field. 

1. A magnetic beta-ray spectrometer comprising a collimating magnetic lens, a radiation source, means for supporting the radiation source adjacent the collimating magnetic lens, said collimating magnetic lens including, electric windings for directing beta-rays which emerge from said radiation source in a parallel beam; a focusing magnetic lens, a detector, means for supporting the detector adjacent the focusing magnetic lens, said focusing magnetic lens including electric windings for focusing the beta-rays onto said detector, the windings of both said lenses being adapted for producing respective focusing magnetic fields, said magnetic fields of both said lenses having respective axial patterns being synchronous and of identical variation, said radiation source and said detector being disposed separately in the respective lens thereof within zones in which the focal length of each respective lens is preserved at a minimum for each axial pattern of each magnetic field; and a magnetic element interconnecting each said lens for dispersing the beta-rays of the parallel beam according to energies of said parallel beam and directing the dispersed beta-rays onto the detector of the focusing lens.
 2. A beta-ray spectrometer according to claim 1, wherein the respective windings each includes a plurality of separate sections electrically insulated from each other, said windings being disposed along the optical axis and further including a plurality of conductive layers electrically insulated from each other, said sections and said layers being coupled to a power supply source for producing a variable axial pattern of the magnetic field. 